Direct Measurement of Electrostatic Forces between Colloids

Abstract

Many applications, such as printing inks and engine oils, are based on dispersions of solid particles in nonpolar solvents. In such systems, electrostatics dominate over other interactions between the particles at large separations, thus controlling their behaviour. Derjaguin–Landau–Verwey–Overbeek (DLVO) theory remains the cornerstone in describing the stability of colloids in dilute, univalent electrolytes. This thesis aims to investigate the electrostatic interactions between charged poly(methyl methacrylate) (PMMA) particles suspended in a nonpolar solvent, dodecane. By probing the double layer surrounding these particles, and testing the stability of the suspensions under various conditions, unexpected observations were made that contradict DLVO theory.

Firstly, the electrostatic repulsions between the particles were directly measured
using blinking optical tweezers. The first significant deviation from expected
behaviour was observed in the trend of the decay of these repulsions with the
electrolyte concentration. By simultaneously measuring the conductivity of the
solutions, the Debye-Hückel (DH) length could also be measured. According to
the DLVO theory, the decay length of the pair interactions should equate with the bulk determined DH length, however our measurements have shown that this is not the case at high electrolyte concentrations. Not only this, but we have measured a nonmonotonic relationship of the decay length of the interactions with the electrolyte concentration, despite the system remaining in the limits of mean-field theories. The second unusual observation made in this system was the formation of aggregates, even at low electrolyte concentrations where the electrostatic repulsions are still strong. Furthermore, the attraction clearly present in the system is strong enough to overcome the steric stability inferred to the particles by a polymer brush layer on their surface. This attraction is not believed to be due to van der Waals interactions, which are too weak and short ranged to overcome the steric stability. By observing the behaviour of the aggregates in an electric field, it strongly suggests that they have a dipole moment, indicating that a dipole interaction could be the origin of attraction.

These observations both provide evidence that the surface of the particles can
charge regulate. By deriving a model to describe the interaction between two charge regulating surfaces, both the unexpected form of the repulsions, and the presence of a dipolar attraction are explained.

Date of Award	1 Oct 2019
Original language	English
Awarding Institution	University of Bristol
Supervisor	Paul Bartlett (Supervisor) & Julian Eastoe (Supervisor)